Interactions between ceria (CeO2) and supported metals greatly enhance rates for a number of important reactions However, direct relationships between structure and function in these catalysts have been difficult to extract because the samples studied either were heterogeneous or were model systems dissimilar to working catalysts We report rate measurements on samples in which the length of the ceria-metal interface was tailored by the use of monodisperse nickel, palladium, and platinum nanocrystals We found that carbon monoxide oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts, clarifying the pivotal role played by the support

http://science.sciencemag.org/content/sci/341/6147/771.full.pdf

Control of Metal Nanocrystal Size Reveals Metal-Support Interface Role for Ceria Catalysts

To improve fuel efficiency, advanced combustion engines are being designed to minimize the amount of heat wasted in the exhaust. Hence, future generations of catalysts must perform at temperatures that are 100°C lower than current exhaust-treatment catalysts. Achieving low-temperature activity, while surviving the harsh conditions encountered at high engine loads, remains a formidable challenge. In this study, we demonstrate how atomically dispersed ionic platinum (Pt2+) on ceria (CeO2), which is already thermally stable, can be activated via steam treatment (at 750°C) to simultaneously achieve the goals of low-temperature carbon monoxide (CO) oxidation activity while providing outstanding hydrothermal stability. A new type of active site is created on CeO2 in the vicinity of Pt2+, which provides the improved reactivity. These active sites are stable up to 800°C in oxidizing environments.

http://science.sciencemag.org/content/sci/358/6369/1419.full.pdf

Activation of surface lattice oxygen in single-atom Pt/CeO2 for low-temperature CO oxidation

The prevailing view of CO oxidation on gold-titanium oxide (Au/TiO 2 ) catalysts is that the reaction occurs on metal sites at the Au/TiO 2 interface. We observed dual catalytic sites at the perimeter of 3-nanometer Au particles supported on TiO 2 during CO oxidation. Infrared-kinetic measurements indicate that O-O bond scission is activated by the formation of a CO-O 2 complex at dual Ti-Au sites at the Au/TiO 2 interface. Density functional theory calculations, which provide the activation barriers for the formation and bond scission of the CO-O 2 complex, confirm this model as well as the measured apparent activation energy of 0.16 electron volt. The observation of sequential delivery and reaction of CO first from TiO 2 sites and then from Au sites indicates that catalytic activity occurs at the perimeter of Au nanoparticles.

http://science.sciencemag.org/content/sci/333/6043/736.full.pdf

Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on a Au/TiO2 Catalyst

Identification and characterization of catalytic active sites are the prerequisites for an atomic-level understanding of the catalytic mechanism and rational design of high-performance heterogeneous catalysts. Indirect evidence in recent reports suggests that platinum (Pt) single atoms are exceptionally active catalytic sites. We demonstrate that infrared spectroscopy can be a fast and convenient characterization method with which to directly distinguish and quantify Pt single atoms from nanoparticles. In addition, we directly observe that only Pt nanoparticles show activity for carbon monoxide (CO) oxidation and water-gas shift at low temperatures, whereas Pt single atoms behave as spectators. The lack of catalytic activity of Pt single atoms can be partly attributed to the strong binding of CO molecules.

https://science.sciencemag.org/content/sci/early/2015/09/02/science.aac6368.full.pdf

Identification of active sites in CO oxidation and water-gas shift over supported Pt catalysts

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites.

Kinetic, isotopic, and infrared studies on well-defined dispersed Pt clusters are combined here with first-principle theoretical methods on model cluster surfaces to probe the mechanism and structural requirements for CO oxidation catalysis at conditions typical of its industrial practice. CO oxidation turnover rates and the dynamics and thermodynamics of adsorption−desorption processes on cluster surfaces saturated with chemisorbed CO were measured on 1−20 nm Pt clusters under conditions of strict kinetic control. Turnover rates are proportional to O2 pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O2 activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C16O−18O2−16O2 reactions, and with infrared bands for chemisorbed CO that did not change within a CO pressure range that strongly influenced CO oxidation turnover rates. Density functional theory estimates of r...

Chemisorption of CO and mechanism of CO oxidation on supported platinum nanoclusters.

One strategy for removing pollutants from diesel engine exhaust is to trap the unburned carbon soot and then to combust the soot with the NO2 that is generated from NO; the two pollutants are then converted to N2 and CO2. Diesel exhaust is relatively cold, compared to gasoline engine exhaust, and conversion of NO to NO2 has required the use of platinum catalysts. W. Wang et al. (p. [832][1]) now report that a more earth-abundant catalyst, based on Mn-mullite (Sm, Gd)Mn2O5 metal oxides was able to oxidize NO in simulated diesel exhaust at temperatures as low as 75°C. Spectroscopic studies and quantum chemical modeling suggested that Mn-nitrates formed on Mn-Mn dimer sites were the key intermediates responsible for NO2 formation.

 [1]: /lookup/doi/10.1126/science.1225091

https://science.sciencemag.org/content/sci/337/6096/832.full.pdf

Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn2O5 for NO Oxidation in Diesel Exhaust

An emission control catalyst for treating an engine exhaust includes non-precious metal group (“NPGM”) mixed phase oxide catalyst having a mullite phase containing optionally in close contact with other metal oxides. The mixed phase catalyst may be included in one or more layers or zones of a multi-layered or multi-zoned emission control catalyst and optionally in combination with precious metal catalysts such as Pt, Pd and Au.

Mixed phase oxide catalysts

A supported palladium-gold catalyst is produced under mild conditions using a commonly available base, such as sodium hydroxide (NaOH) or sodium carbonate (Na2CO3). In this method, support materials and a base solution are mixed together and the temperature of the mixture is increased to a temperature above room temperature. Then, palladium salt and gold salt are added to the mixture while maintaining the pH of the mixture to be greater than 7.0 and keeping the mixture at a temperature above room temperature. This is followed by cooling the mixture while adding acetic acid to maintain the pH of the mixture to be within a desired pH range, filtering out the supported palladium-gold particles, washing with a pH buffer solution and calcining.

Palladium—gold catalyst synthesis

Supported catalysts are produced with nanometer sized particles comprised of different metals dispersed throughout the catalyst support material. The supported catalysts reduce substantially or completely the amount of platinum that is required without sacrificing catalytic performance. In place of platinum, the supported catalysts employ palladium, silver, or copper, all of which costs significantly less than platinum.

Xianghong Hao

Papers

Chemisorption of CO and mechanism of CO oxidation on supported platinum nanoclusters.

Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn2O5 for NO Oxidation in Diesel Exhaust

Mixed phase oxide catalysts

Palladium—gold catalyst synthesis

Method of producing multi-component catalysts